1. Field of the Invention
The present invention relates to an inkjet image forming apparatus, and more particularly to technology for improving image formation quality (image quality) in an inkjet image forming apparatus based on a single pass method which is equipped with an inkjet head having nozzles in a two-dimensional configuration.
2. Description of the Related Art
In the field of inkjet image formation, an inkjet image formation method (single pass method) is known in which, in order to achieve high image formation resolution and high productivity, head modules comprising a plurality of nozzles arranged in a two-dimensional configuration are formed, a long head (known as a “page-wide head” or “full line type head”) which covers an image formation area spanning the entire width of the paper is composed by aligning a plurality of sub-heads which are constituted by the head modules, in the paper width direction (hereinafter, called the “x direction”), and an image is formed on the paper by performing just one relative scanning action of this long head and the paper in a direction (hereinafter, called the “y direction”) which is perpendicular to the x direction.
A single-pass composition of this kind employs relative movement of the head and paper (a paper conveyance system which holds and conveys the paper), and therefore the head and the paper are not unified (in a fixed positional relationship) and relative displacement or vibration may occur in directions other than the relative scanning direction (y direction) during the image formation process. The causes of this relative displacement and vibration include, for instance, various mechanical shocks caused internally and externally to the image forming apparatus, displacement caused by the drive system for driving various moving parts including the paper conveyance system, and so on, and such factors manifest themselves as relative vibrations between the head and paper. Of the relative vibration between the head and the paper, the vibration in the x direction in particular generates non-uniformity which causes problems of image quality in a two-dimensional nozzle arrangement.
In relation to relative vibration of the head and the paper, Japanese Patent Application Publication No. 10-235854 discloses technology for reducing image abnormalities (band-shaped “vertical stripes” extending in the paper conveyance direction (y direction)) which are caused by abnormal ejection dots, by oscillating or moving a head in a direction (x direction) perpendicular to the y direction, in an inkjet apparatus based on a single pass method employing a line head having one-dimensional arrangement of nozzles.
The apparatus composition in Japanese Patent Application Publication No. 10-235854 prevents, due to its one dimensional nozzle arrangement, problems of image quality caused by relative vibration and relative movement of the head and the paper (recording paper) in the x direction and achieves a reduction in non-uniformity by using other nozzles to compensate for recording of missing dots by making active use of the vibration in the x direction. However, in the case of a two-dimensional nozzle arrangement, as described hereinafter, a major problem which is characteristic of a two-dimensional arrangement occurs.
3. Description of Technical Problem
In a head having a two-dimensional nozzle arrangement, of the pairs of nozzles which form dots that are mutually adjacent in the x direction on the paper (or a raster created by linking dots continuously in the y direction), there are nozzle pairs which are in a positional relationship separated by a distance in the y direction, in the layout of nozzles in the head (such nozzles are called a “y-offset adjacent nozzle pair” below).
In this case, if there is relative vibration in the x direction between the head and the paper, then the pitch between the rasters recorded by the y-offset adjacent nozzle pair varies depending on the relative vibration. As a result of this, a “weighting (overlapping)” or “gap” appears between the dots (adjacent dots in the x direction) which are recorded by the y-offset adjacent nozzle pair, and the extent of this “weighting” or “gap” changes in the y direction, producing a non-uniformity which degrades the image quality. In the present specification, density non-uniformity which is caused by relative vibration or displacement in the x direction between the paper and a head having a two-dimensional nozzle arrangement in this way is called “vibration non-uniformity”.
A phenomenon of this kind is described here by means of the examples in FIG. 18 to FIG. 23. FIG. 18 is one example of a two-dimensional nozzle arrangement. A black dot “●” in FIG. 18 indicates a nozzle position. The horizontal axis represents a position in the x direction and the vertical axis represents a position in the y direction; a nozzle position is represented by coordinates in pixel (pix) units which are determined by the recording resolution.
As shown in FIG. 18, this two-dimensional nozzle layout has two nozzle rows separated in the y direction, and within the same row, nozzles are arranged every other 1 pix (the x-direction nozzle pitch within one row is 2 pix) and the positions of the nozzles belonging to different rows are staggered by 1 pix in the x direction with respect to each other (a so-called staggered matrix configuration). As a result of this, an image formation mode is adopted in which, a raster (scanning line) is formed on the paper every other 1 pix by the nozzle group belonging to the first row, and rasters formed by the nozzle group of the second row are embedded between the rasters formed by the nozzles of the first row. The pitch in the y direction between the first and second rows is called the offset amount of the “y-offset adjacent nozzle pair” (y-direction offset amount). Here, an example is given in which the y-direction offset amount is 500 pix. If the image formation resolution is 1200 dpi, then 500 pix is 10.6 mm.
Regarding a head having a two-dimensional nozzle arrangement as shown in FIG. 18, FIG. 19 shows one example of rasters drawn by respective nozzles in a case where there is relative vibration in the x direction between the head and paper. FIG. 19 shows a group of rasters obtained when ejection is started simultaneously from all of the nozzles and continuous ejection is performed at a prescribed droplet ejection frequency while conveying the paper at a uniform speed in the y direction. Furthermore, FIG. 20 shows an example of an image actually formed on paper in this case (a solid image; droplet ejection rate 100%). FIG. 19 and FIG. 20 are examples of a case where the single amplitude (half amplitude) of the relative vibration in the x direction is 5 μm, and the period of the relative vibration is 1000 pix=21.2 mm when converted to a spatial distance on the paper in the y direction.
In FIG. 19, the raster indicated by reference numeral 1A is drawn by nozzles belonging to the lower row (first row) in FIG. 18. In FIG. 19, the raster indicated by reference numeral 2B is drawn by nozzles belonging to the upper row (second row) in FIG. 18. The raster 1A and the raster 2B are separated by the equivalent of 500 pix in the y direction. This corresponds to the y-direction offset amount between the lower row nozzle and the upper row nozzle in FIG. 18.
If it is supposed that there is no relative vibration in the x direction between the head and the paper, then the scanning lines (rasters) of the y-offset adjacent nozzle pair are straight lines which extend in perfectly straight fashion in the y direction, and the pitch between the rasters is a uniform value determined by the resolution (for example, a pitch of about 21.2 μm in the case of 1200 dpi resolution).
On the other hand, if there is relative vibration in the x direction between the head and the paper, then the raster of a nozzle of the first row (reference numeral 1A) and the raster of a nozzle of the second row (reference numeral 2B) fluctuate respectively (see FIG. 19). This fluctuation of the rasters causes variation in the spatial period of the x-direction pitch between mutually adjacent rasters (1A, 2B), depending on the position in the paper conveyance direction (y direction).
As a result of this, as shown in FIG. 20, periodic non-uniformity occurs in the resulting image that is formed. More specifically, since the x-direction pitch between rasters which are mutually adjacent in the x direction varies periodically, then a “weighting” of the adjacent rasters (mutual approach of the rasters) and a “gap” in the adjacent rasters (distancing of the rasters) are repeated in the y direction, and this appears as a density non-uniformity in the image formation results on the paper.
In FIG. 20, a white-striped region 4 in which white stripes extending in the y direction are arranged roughly equidistantly in the x direction, and a black region 5 where the white stripes are interrupted in the y direction and appear darker (more dense) are repeated at ½ of the cycle of the vibration in the y direction (here, 500 pix).
Looking across the white-striped region 4 in the x direction, a portion where there is a white gap (white stripe) and a portion where there is no white stripe (black portion) are repeated alternately. If the white-striped portions are viewed in further detail, the gaps between white stripes (the thickness of the white stripes) are not uniform in the y direction, but rather become larger in the central portion. If the white-striped region 4 of this kind is viewed macroscopically, the density is reduced compared to the black region 5, and therefore when the image is viewed as a whole, a density non-uniformity is visible in which the density varies in the y direction (dark/light shading is repeated periodically), and therefore image quality declines.
In the description above, an example is given in which nozzles are arranged two-dimensionally in 2 rows (y direction) by N columns (x direction, where N is an integer and N≧2), but the present problem is not limited to this nozzle arrangement and a similar problem occurs in other two-dimensional nozzle arrangements (for example, an M row×N column two-dimensional nozzle arrangement, where M is an integer and M≧2).
FIG. 21 shows a case of a nozzle layout having 6 rows by N columns. Similarly to FIG. 18, if the half amplitude of the relative vibration is 5 μm, then the period of the relative vibration is 1000 pix=21.2 mm in terms of a y-direction distance on the paper. FIG. 22 shows one example of rasters in a case where there is relative vibration in the x direction between the head and the paper, in a head having the nozzle arrangement in FIG. 21, and FIG. 23 is an example of an image (solid image) formed in this case.
In the case of the nozzle arrangement shown in FIG. 21, there are a total of six combinations of nozzle rows having nozzles which constitute y-offset adjacent nozzle pairs: the first row and second row, the second row and third row, the third row and fourth row, the fourth row and fifth row, the fifth row and sixth row, and the sixth row and first row. Density non-uniformity occurs due to variation in the pitch between the rasters corresponding to these respective nozzles (see FIG. 23), and of this non-uniformity, the white stripes caused by variation in the pitch between rasters formed by the pair of nozzles which are spaced furthest apart in the y direction (namely, the nozzles of the sixth row and the nozzles of the first row) are most conspicuous and this nozzle pair which have the largest offset amount have the greatest effect on image deterioration.
In this case, as shown in FIG. 23, the white-striped region 6 and the black region 7 are repeated at a vibration period (here, 1000 pix) in the y direction. In FIG. 20 and FIG. 23, the period of the vibration non-uniformity (white-striped region and black region) varies due to the following reason.
The nozzle arrangement in FIG. 20 involves an alignment of two rows as shown in FIG. 18. In this case, there are two sets of “y-offset adjacent nozzle pairs”, namely, a set of “first row nozzle—second row nozzle” (hereinafter called “A set”) and a set of “second row nozzle—first row nozzle” (hereinafter called “B set”). A vibration non-uniformity having a vibration period (1000 pix) occurs in the A set nozzle pair and a vibration non-uniformity having a vibration period (1000 pix) occurs in the B set nozzle pair. Since the vibration non-uniformities created by the two sets of nozzle pairs are mutually displaced by 180 degrees in terms of the phase, then the synthesized vibration non-uniformity has a period (500 pix) of ½ of the vibration period (see FIG. 19).
On the other hand, the case shown in FIG. 23 corresponds to the nozzle arrangement indicated in FIG. 21 (a six-row arrangement), but in this case, the “y-offset adjacent nozzle pair” is formed by only one set: “sixth row nozzle—first row nozzle”, and the period of the vibration non-uniformity which appears is the vibration period (1000 pix) only (see FIG. 22).